Problem restarting after short break

The B&S 675 engine on my Toro lawn mower starts easily and runs well while I cut my front yard. Then I take a 5 min. brake when the engine is off. When I try to restart, it will not. I have to wait about 10-15 min more and then it will start. The plug, oil, filter, are all new and the gasoline is fresh. What else could it be?